The present invention relates to a magnetic resonance image synthesizing system, and in particular to a magnetic resonance synthesizing system capable of displaying an arbitrary region of interest clearly.
Blood vessel imaging methods using magnetic resonance images (hereafter abbreviated to MR images) have been proposed as described in "Method for MR Angiography", Society of Magnetic Resonance in Medicine, VOL. 1 (1985), pp. 591 to 592, for example. In one method, images are obtained by using a pulse sequence for extracting signals from a moving region of a human body. In another technique, MR images in an expansion interval of a heart and MR images in a contraction interval of the heart are derived and a differential image is produced from the difference between those images. These methods are capable of displaying only moving portions. When flow velocities are different at respective portions in a case such as blood flow, for example, however, it is difficult to obtain clear images of a region of blood vessel of interest.
On the other hand, relaxation time enhanced images which are displayed with the difference between relaxation time values enhanced provide information which is important for clinical determination with respect to an affected part. In many cases, therefore, the images with enhanced relaxation time provide more important significance than spin density images representing morphological information. These images with relaxation time enhanced are obtained with different contrasts by changing scanning parameters such as the duration of irradiation of an RF pulse in the pulse sequence and the interval of its irradiation. For obtaining a desired image, therefore, it is necessary to obtain several images while changing scanning parameters, resulting in a problem of extremely long scanning time. Therefore, there has been proposed a scheme in which an image under an arbitrary scanning condition is synthesized on the basis of information of a plurality of images with relaxation time enhanced which were scanned beforehand.
In a method described in "Clinical Evaluation of Synthetic MR Images in Intracranial Diseases", SMRM '86, VOL. 3, pp. 762 to 763 (1986), for example, the spin density, the spin-lattice relaxation time and the spin-spin relaxation time are calculated from a plurality of images with relaxation time enhanced which were scanned beforehand. The calculated values are substituted into a signal strength calculating expression to calculate images with relaxation time enhanced at the time when the scanning parameters are arbitrarily changed.
Further, there also has been proposed a scheme in which an arbitrary synthetic image is directly calculated by computation of a plurality of images with relaxation time enhanced which were scanned beforehand.
Especially when an image representing only blood vessels is synthesized by using the former prior art, the phase rotation angle of the spin changes depending upon the blood velocity. Accordingly, the relationship between the phase angle of the resultant measured signal and the blood velocity is fixed in accordance with the scanning sequence. Even if there is a region of interest, therefore, the velocity of that region is unknown and hence a problem that the region is not necessarily displayed clearly.
Further, in the latter prior art, the scheme for calculating images with relaxation time enhanced is considered for a case where an image with relaxation time enhanced is to be synthesized. However, a synthesis and display scheme with due regard to man-machine interface is not shown.